Liquid level sensors are used in a variety of applications to sense fluid levels in reservoirs where it is important or desirable to periodically or continuously measure the level of fluid within the reservoir.
Prior art sensors include conventional fuel sensors which include a float carried on the end of a long pivoting arm suspended within the interior of the fuel supply tank. As the arm pivots, a short wiper blade having a ball contact slideably engages wires on a wound wire rheostat that varies in resistance in accordance with the position of the float and hence liquid level. The angular position of the pivoting float arm, or mote particularly the incremental angle of the arm, is not directly proportional to the vertical incremental change in float position and hence not directly proportional to liquid levels. An improvement in this type of mechanism is disclosed in U.S. Pat. No. 4,920,798 issued to Weaver. This reference teaches a fuel level sender which has a vertically reciprocal foam float that carries a slideable dual dwell contact member biased by springs in the float toward a fixed film resister plate.
Another form of liquid level sensor employs a capacitive probe having a pair of continuous elongated elements positioned on a substrate of the probe. This form of sensing system makes use of the difference in the dielectric of air from various liquids. In such systems, some means is provided for generating a signal which is applied to one plate of the probe. The overall capacitance of the capacitor formed by the two plates, and thus the magnitude of the signaled coupled onto the other one of the plates on the probe, will change as the percentage of the probe submerged in a fluid changes. Thus, the magnitude of the signal coupled onto the output plate of the probe can provide a relative indication of the area of the probe which is submerged in fluid and/or exposed in air. An example of this type of sensor is shown in U.S. Pat. No. 5,477,727 issued to Koga.
While the prior art devices have proven effective in certain applications, various problems exist with each style of sensor. The more mechanical-type sensors have proven less reliable and difficult to use when the space required for the sensor must be minimized. Therefore, in many applications, the use of the mechanical device is not an option. In contrast, the capacitive sensors require less space. Capacitive-type sensors rely on the difference of the dielectric constant of fluid and air. In order to measure this difference, the fluid flows between two capacitive plates. As the plates must be spaced proximate to each other, the flow of the fluid may be constricted if the fluid has debris or the like provided therein. In other words, the debris may lodge between the plates causing the flow of liquid to be restricted. Also, a false reading may occur due to the different dielectric constant of the foreign object. Another problem with the capacitive sensor also relates to the confined space between the plates. As different levels have different viscosity, the speed by which the liquid will flow in confined spaces will vary significantly. If the sensor is to be used with less viscous fluid, the sensor may sense the level improperly as the liquid captured between the plates will move at a different speed than the remaining fluid in the tank or reservoir.
Electronic sensors generally require three-wire installation (power, ground, and signal). While this type of configuration is practical in new installations, it is difficult to use three-wire installation to replace mechanical-style sensors which use two wires. Therefore, the use of prior art electronic sensors is limited in many applications.
It would, therefore, be beneficial to provide an electronic sensor which can be used as a replacement for mechanical and electrical sensors. It would also be beneficial to provide a sensor which can be used with various liquids, without concern of the viscosity or relative purity of the liquid.